In recent years, plastic optical fibers have enjoyed great success in applications which demand miniaturization of components and reduction of cost. One area of increased utilization is in sensors and instrumentation for medical applications, as for example in PO.sub.2 and pH probes, where mechanical flexibility, durability and low cost are critical factors.
In designing such fiber optic sensors and related instrumentation, it has become extremely desirable to have a method of directly joining plastic optical fibers. For example, in typical PO.sub.2 probes, generally the terminal ends of two fibers are positioned in side-by-side parallel alignment so that excitation light enters the probe from one fiber and luminescent light from the probe passes into the other fiber. Improved performance, optically and mechanically, however, could be achieved if the fiber ends were physically joined together rather than merely being held side-by-side. Further in conventional PO.sub.2 and pH probes, where two wavelengths of light are required for measurement, a bifurcated or joined fiber design would permit the sensor to be carried on only a single fiber. Thus, an even smaller sensor could be achieved than that which is presently used.
At present, various methods are known for joining or bonding optical fibers, and may be categorized as end-to-end connections and as side-by-side connections.
End-to-end connections are typified by the following U.S. Patents: BUHITE et al (U.S. Pat. No. 3,810,802) which discloses a method of joining together single optical fibers collinearly aligned within a sleeve by filling the space remaining in the sleeve interior at the junction of the fibers with a low melting point transparent thermoplastic and then applying heat to melt the thermoplastic; ROWE et al (U.S. Pat. No. 3,928,102) which discloses a method of joining two optical fibers end-to-end using heat and/or an adhesive where the fibers are supported, for alignment, by surface tension of a liquid; MATSUNO (U.S. Pat. No. 4,196,965) which teaches connecting a pair of optical fibers end-to-end by stripping away the plastic cladding surrounding the fiber ends, aligning the ends, heat melting the abutting ends and then covering the ends within a laminated sleeve; and KERSTEN (U.S. Pat. No. 4,220,619) which discloses the use of a heat-shrinkable tube to secure first and second groups of fibers end-to-end.
Side-by-side connections are typified by HUDSON (U.S. Pat. No. 4,083,625) and LEMESLE et al (U.S. Pat. No. 4,256,365) both of which disclose a method of joining a pair of fibers in which adjacent cladding of the side-by-side fibers is fused. HUDSON further discloses fusing the end faces of the joined pair to a single fiber. KAWASAKI et al (U.S. Pat. No. 4,291,940) teaches connecting a pair of fibers positioned side-by-side by fusing the fibers along a small portion of their length.
None of the above references disclose or teach side-by-side connection of two or more plastic optical fibers by fusion of the fiber ends. The methods previously described for glass fibers are not suitable or applicable to plastic fiber joining. Such a procedure, heretofore has not been practiced or effected, and heretofore the notion of joining fiber ends side-by-side by fusion has been shelved by anticipation of exceedingly high costs in designing and engineering specialized fixtures which might be required to hold the fibers ends in alignment prior to effecting a connection by heat-melting or fusing of the fibers themselves.
There is therefore a clearly defined need for a method of joining two or more optical fibers side-by-side which overcomes the above deficiencies.